1. Field of the Invention
This invention relates to a coordinate input apparatus, and more particularly to a vibration transmission plate for inputting coordinates used in an apparatus which detects elastic vibration wave input from a vibration input pen by a plurality of sensors provided at the vibration transmission plate. The apparatus also detects the coordinates of the input position of vibration by the vibration input pen based on the transmission time of elastic vibration input from the vibration input pen to the vibration transmission plate.
2. Description of the Related Art
Various types of coordinate input apparatuses for inputting handwritten characters, drawings or the like to an information processing apparatus, such as a computer or the like, have been proposed. Among these types, a type which utilizes the transmission of the vibration of an ultrasonic wave and calculates coordinates by measuring the transmission delay time of the wave has been known to have a simple structure and high reliability. Since a vibration transmission plate can be made of a transparent material, such as plastic, glass or the like, it is possible to provide an information input apparatus, which can be used with high operational feeling as if the user is writing an image on paper, by superposing the vibration transmission plate on a liquid crystal display or the like. The vibration transmission plate used in this type is required to transmit the vibration of the ultrasonic wave input by the vibration input pen to sensors as a plate wave under various kinds of operational environment of the user.
In selecting a material for the vibration transmission plate, glass can be effectively used from the viewpoint of its high transparency and small damping coefficient for vibration. However, if the vibration transmission plate comprises a single glass plate, the detected position changes when the vibration input pen is inclined in the above-described state of using the coordinate input apparatus. This change is caused by a change in the phase of the plate wave propagated in the vibration transmission plate due to a change in the force vector of the vibration input to the glass in accordance with the inclination of the vibrating input pen. As a result, the arrival time of the wave detected by a sensor changes, causing a decrease in accuracy of the detected position.
FIG. 1 is a diagram illustrating the configuration of a model used in order to confirm the performance of the vibration transmission plate. This model comprises a uniform vibration transmission plate made of a single glass plate. In FIG. 1, sensors 6A and 6C are provided at left and right positions relative to the input position of vibration input pen 3 so as to have the same distance therefrom. The two sensors and the input position of vibrating input pen 3 are on the same cross section. FIG. 2 is a diagram illustrating the behavior of vibration waves received by sensors 6A and 6C when vibrating input pen 3 contacts the vibration transmission plate with angles of 90.degree. and 60.degree., with the abscissa representing time and the ordinate representing the amplitude of the wave.
In FIG. 2, the vibration waves received by sensors 6A and 6C when vibrating input pen 3 contacts the vibration transmission plate with angles .theta.=90.degree. and .theta.=60.degree. are indicated by a broken line and solid lines, respectively.
The vibration waves received by sensors 6A and 6C differ by inclinating vibrating input pen 3, and time delays occur at points where the vibration waves cross the line representing the amplitude value=0, as indicated within circles shown in FIG. 2. It is known that the time delay, that is, the phase delay .epsilon. of the vibration wave occurs when the vibration transmission plate comprises a single layer of a uniform material, which is not limited to glass. If the phase delay .epsilon. has a large value, an error in position detection increases when position recognition is performed by an arithmetic and control circuit using signals representing the vibration waves received by sensors 6A and 6C which pass through a signal waveform detection circuit.
In order to remove the influence of the inclination of the vibration input pen, vibration transmission plates having a multilayer structure have been proposed. However, materials for the vibration transmission plate and the configuration of the multilayer structure have not been deeply considered, and there is still room for improvement in accuracy of the detected position.